Mini-roundabouts at Crossroads

My apologies to overseas viewers who can install mini-roundabouts of any size. Here in the UK we have a rule that limits the size of the central island to just 4m diameter. This has caused serious problems and lost many potentially useful sites. 
Some authorities will not now install mini-roundabouts at crossroads as a matter of policy.
So UK designers will be particularly interested in the crossroads at Binfield where I ignored this rule in 1988 and installed one of the most successful schemes ever (images below). Here the central island is about 6m diameter. I have recommended a mini-roundabout at a crossroads where the central island would be nearly 10m to achieve the necessary deflection at a site where the ICD is about 23.5m, too small for a solid central island.
Some Drawings of idealised layouts to illustrate deflection problems...
Here is a series of five drawings of symmetrical idealised crossroads. Each illustrates central island sizes required to provide deflection at 60m radius for approx. 2m wide vehicle paths. The island size is determined by the deflection required which in turn is determined by ICD and other factors such as road widths on entry and exit, flare, etc. In this series of drawings with reducing ICDs the first is the only one with a solid centre. In the following four layouts it is only the truck apron which remains, and it is always designed to be large enough to deflect light vehicles, and that will often mean a diameter larger than 4m.
1. Small Roundabout

What we are seeing in effect here are two roundabouts! One designed for trucks, coaches etc. and one designed for light vehicles.

In the UK this would be legal.

The size of the truck apron here is determined mainly by the road widths. Reducing these locally would help pedestrians.
(See Millennium Vision page)

2. Larger mini-roundabout

There are many cross-roads like this in the UK and thousands in America.

To deflect traffic by the central island alone requires quite a large island. Deflection would be assisted here by the use of build-outs on the approaches but avoid significant reduction of the ICD.

In my seminars we look at a possible way of dealing with one of the sites at Hove which is remarkably similar.

In the UK this might be illegal!
or require special authorisation.

3. Large crossroads

This is a relatively extreme case where a mini-roundabout might be tried on a crossroads with tight corner radii and wide approach roads. As with the previous layout narrowing the approaches will make a better layout altogether but which may still require over 4m for the central island.

Again, this might be illegal in the UK!

4. Medium-size flared crossroads

7.3m is the typical width of many UK roads so this illustrates well the need for a central island larger than 4m. Many highway authorities will not now install mini-roundabouts in these circumstances which is a pity because they can work well provided that there is sufficient deflection, achievable using a larger central island. In addition I would recommend raised splitter islands at a site like this.

Still technically illegal in the UK!

5. Small crossroads - little flare

I would not normally recommend a mini-roundabout for this geometry unless speeds are already very well controlled by other means than the mini-roundabout lateral deflection. This drawing is really to illustrate the position compared with the other ones.

In the UK such a mini-roundabout would be legal (but hardly practical).

It was seeing the Americans using extensive truck aprons that convinced me of the need for re-assessing what a mini-roundabout central island is and does. This is summarised below:

In effect a mini-roundabout is a truck apron without a solid centre.

Here is a drawing of a symmetrical layout based on a typical crossroads
(Note: the overrunnable splitter islands may now be illegal in the UK.
One illuminated bollard on each guarantees legality.)

Note the larger overrunnable central island with shallow kerbs, raised but nearly flat;
Overrunnnable splitter islands with similar kerbs;
Approaches are below 4m wide so will be single lane only;
Signs will have to be on the nearside, but place on the straight;
Hatching and arrows not shown.

Special authorisation required in the UK.

* * * * *

I urge all UK authorities to do two things:

1. Examine all of your existing crossroads mini-roundabouts and see if they could be improved
by enlarging the central island and associated alterations;
2. Look out for poorly performing priority junctions or signal control at crossroads and
see if this sort of solution might work better.
A word of caution...
Don't try to square up an obviously staggered or scissored crossroads to install a single mini-roundabout; install a double mini-roundabout instead.

Binfield Crossroads, Bracknell, Berkshire

Here are three views of Binfield Crossroads which is very close to layout 4 above. This has an exceptionally good safety record of just one slight injury accidents since 1/1/1991! The crucial factors:

  • Deflection provided by large raised central island

  • Split on approaches into 2 narrow lanes

  • Signs very well placed

  • Lots of turning traffic

Other options: I would look now at the street lighting which seems inadequate. Lamp columns would be better placed on each corner such that four units would illuminate the junction. Standard lighting drawings fail to do this and can be dangerous as the layout can give the impression of continuity across the junction "hiding" the crossroads.

View from the west. The crucial lane-split is now worn out and should be refurbished. The central island is about 6m diameter although it looks smaller.

View from the east. The red surfacing under the hatching is new. I would change the splitter island colour infill and renew the lane-split markings. Note the use of the off-side only mini-roundabout sign (dia 611.1).

A closer view of the central island which too needs refurbishing.
Had this been just 4m there would have been
straight through paths.

1. Deflection I insisted that the central island had to be large enough to deflect the four crossing movements. This meant about 6m diameter. It is slightly over-height but is certainly a deterrent. The ICD is around 20m so the central island has to be over-runnable. I now recommend a shallow kerb for these larger central islands.
2. Flows There are plenty of turning movements at the junction - so over time all drivers have learnt high expectation of the need to give way/yield. The former side-roads carry significant traffic volumes.
3. Two-lane approaches The approaches from the former major road are split into two narrow lanes - for drivers this is a powerful visual tool just in case they missed everything else.

Halfway St/Willersley Ave, Bexley (Nov 2004)

Here is a plan which illustrates exactly the problem at a real site. Formerly a crossroads, the roundabout will involved some widening (the blue lines) but not to affect the BT boxes (yellow). Existing refuges are shown with my larger ones superimposed. ICD approx. 20m. The brown areas are overrunnable, the central island is just over 7m to the edge with approx 600mm horizontal climb to a height of 50-75mm. Quite a lot of buses turn from E->N & vice versa.

Clearly, for this scheme to be safe a 4m centre would be insufficient allowing drivers to "straight-run" it.

INSTALLED - 5 June 2005 - working well - images on
(Sorry, I STILL have not been to the site; as soon as I have more images I will post them.)

Belt Road/ChurchBrae, Derry
Double Mini-roundabout in widened outline

This scheme illustrates a design for a straight crossroads in Derry, not right-angled, but with around 50 between the axes. One of the earliest schemes at Upton Cross in Dorset was similar, installed in 1970 on the (then) A35/A350 intersection previously controlled by traffic signals. The double mini-roundabout always ensures better deflection than a single mini-roundabout. In any case the latter at such a crossroads would fail with drivers making the acute right turns unable to pass to the correct side of one central island. Designers should not be afraid to use designs like this; they are particularly effective and very safe.

Other examples: Arch Hill crossroads Truro (Mini-roundabouts - A Definitive Guide Fig 5, p22), Maidstone Road, Wigmore Road, Gillingham, Kent (photo 10, p7).

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Penntraff - Dec 2014
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